A window hole of the gears having window holes are conventionally formed by forming a circular hole by drilling and reamer process, by boring by milling cutter and broaching or by blanking by press.
Drilling and reamer processes need a much greater expense for providing equipment for these processes and consumable tools and the forms for the window holes are limited to circular holes. Therefore, the degrees of freedom of the diameter of a circular hole process and so on are restricted according to the diameter of a clutch gear.
Moreover, because the clutch gear and the circular hole are formed in different processes, it is difficult to obtain accuracy of the hole forming phase i.e. the pitch of holes against the clutch gear.
Furthermore, because cutting scraps become attached and weld flashes occur, it is difficult to secure quality and uniformity of products.
In milling cutter and broaching, the forms of window holes used are not limited to circular holes, but those process have problems in that as wear and tear on the equipment occurs and the need for tools specific to cutting, inefficiency, the occurrence of cutting scraps and weld flashes, the difficulty of securing hole phases against the clutch gear and the requirement of providing a clearance for tools used in the processes must be taken into account. Therefore, in those processes, products often have defects in terms of strength.
Compared to these processes, in blanking by press machining, the time required is shorter and the forms of window hole are free to choose from. Therefore, blanking by press is well-suited to mass production.
Window holes formed by blanking by press have a weld flash in a blanking direction. Therefore, it is necessary to rework the gear by cutting in order to delete the weld flash after blanking and the structure of a die for blanking is complex in order to delete the potential for weld flash.
Furthermore, cutting during the reworking causes an interruption in cutting completion. Therefore, the resultant tool life is short, the window holes are limited to being penetrated holes and it is difficult to maintain a uniform tooth form due to the difference in pressure strength between a tooth form formed near by the window hole and a tooth form formed far from the window hole, while blanking.